THE SKULL OF OSSEOUS FISHES. 97 



in the Carp their bases meet, like those of the ali-sphenoids, above tlic 

 sphenoid : when osseous matter is (leveh)pe(l in the intcrorbital .septum 

 the orbito-sphenoids arc articulated by their under and anterior part 

 to that bone or bones.* The olfactory nerves pass out of the skull 

 by the superior interspace of the orbito-sphenoids, and the optic 

 nerves by their inferior interspace, or by a direct perforation ; and 

 the essential functions of the oi'bito-sphenoids relate to the pro- 

 tection of the sides of the cerebrum or prosencephalon, and to the 

 transmission of the optic nerves. The orbito-sphenoids frequently 

 bound or complete the foramen ovale. 



The frontal or mid-frontal bone, (spine of the prosencephalic 

 arch, ib. il), completes the prosencephalic arcli above, as the 

 suiira-occipital does that of the epencephalon ; but it ahvays enters 

 into the formation of the cranial cavity, though its major part 

 forms the roof of the oi'bits, which accessary function is the chief 

 condition of the great expanse of this neural spine in fishes. 

 Single, and sending up a median crest in the Cod, the Ephippus, 

 and some other fishes, the frontal is more commonly divided 

 along the median line, the divisions having the form of long and 

 broad sub-triangular plates ; narrower in the lofty compressed 

 skulls, smaller in those with large orbits, and becoming greatly ex- 

 j^andcd in the fishes with small and deep-set eyes. The frontals rest 

 in a small part of their extent upon the orbito-sphenoids, but are 

 more constantly articulated anteriorly, to the nasal and pre-frontals, 

 and posteriorly, with the post-frontals, the parietals, the mastoids, 

 and frequently also with the supra-occipitals : each frontal sends up 

 its own crest in the Tunny f, the interspace leading to a foramen, 

 penetrating the cranial cavity in front of the single occipital spine : a 

 larger fontanelle exists in the Cobitis and some Siluroids betAveen the 

 fi'ontal and parietal bones. In the Salamandroid fishes (e. g. Pohjptcrus) 

 each frontal sends down a vertical longitudinal plate, which rests 

 directly upon the anterior prolongation of the sphenoid, and inter- 

 cepts a canal along which the olfactory nerves are continued forwards 

 to the prefrontals : the lateral parietes of this canal thus form not 

 only a complete, but a double bony partition between the orbits. \ 

 In the Shad a corresponding descending plate takes the place of 

 the orbito-sphenoid. In most Acanthopteri an olfactory groove is 

 formed by shorter vertical descending plates from the under surface 

 of tlie frontal. The mid-frontal is single in the PlcuronectUhe, but 



* The specially developed inteiorhital sei)tinn or cranial a'thnioid of Ciivier in 

 the Bream and Carp misled Bojamis into the belief that it was the body of the 

 prosencephalic vertebra (vertebra optica). — Isis, 1818, p. 502. 



f Ueminding one of the double spine of the neural arch of the atlas in Tc» 

 trodon. I XXII., t. iv. p. 122. 



VOL. IL II 



